Wireless communications is becoming increasingly important, with wireless systems finding their way into every growing numbers of applications. One consideration in wireless systems is security, and wireless systems can be vulnerable to eavesdroppers and intentional interferers (jammers). Protecting the information transmitted through wireless link is often an important consideration. Other aspects of a wireless communication link can also be important to protect from exploitation. For example, traffic analysis of a wireless communication network may help an adversary to determine what functions different nodes in a network are providing without requiring the adversary to actually decode the content of messages within the network. Radio emissions from a wireless communications link may be used to locate or home in on the node making the emissions. An adversary may also attempt to interfere with (jam) communications links.
In general, communications security (COMSEC) encompasses measures and controls taken to deny unauthorized persons information derived from telecommunications and to ensure the authenticity of such telecommunications. COMSEC can be broken into a number of differing, although overlapping, aspects: cryptosecurity, transmission security, emission security, traffic-flow security, and physical security. Cryptosecurity encompasses protecting the information that is communicated (e.g. data) and typically involves the use of cryptosystems to ensure message confidentiality and authenticity. Emission security (EMSEC) encompasses measures taken to deny unauthorized persons information of value, which might be derived from intercept and analysis of compromising emanations from crypto-equipment, automated information systems (computers), and telecommunications systems. For the purposes of the present discussion, we considered EMSEC to refer to unintentional emissions. In contrast, transmission security (TRANSEC) is the application of measures designed to protect intentional transmissions from interception and exploitation by means other than cryptanalysis. Finally, traffic-flow security includes measures that conceal the presence and properties of valid messages on a network. Finally, physical security encompasses physical measures taken to safeguard equipment, material, and documents from access thereto or observation thereof by unauthorized persons.
Turning to transmission security, TRANSEC has traditionally involved using spread spectrum techniques, such as frequency hopping and direct sequence. Benefits of spread spectrum include spreading the transmission energy over a wider bandwidth, making the signal harder to detect and jam. The frequency-hopping pattern or direct sequence code is typically derived using a pseudorandom sequence. On one hand, the pseudorandom sequence must usually be known by intended receivers to allow the intended receivers to synchronize to the signal. On the other hand, the pseudorandom sequence must be kept secret from would-be interceptors, or many of the benefits of spread spectrum are lost. Thus, traditional wireless networks using spread spectrum typically include a key distribution scheme to allow the spreading sequence keys to be distributed through the network. This key distribution can be cumbersome. Some system use predefined fixed keys to avoid the need for key distribution, but this can make the system vulnerable, as if the key is ever divulged the security is lost.
Another difficulty with spread spectrum systems is that synchronization can be difficult. In order for the receiver to detect the signal in frequency hopping systems, it is necessary for the receiver to frequency hop in synchronization with the transmitter. Thus, the receiver must synchronize to the timing of the transmitter, taking into account any differences in timing that are introduced by propagation delay between the transmitter and the receiver. In direct sequence systems, the receiver must generate a local spread sequence that is time synchronized with that of the transmitter to allow the spreading sequence to be removed. Many different schemes, including the use of external time synchronization sources, preamble sequences, and pilot channels have been developed to address this challenge. Nonetheless, the synchronization requirements often add considerable complexity to a spread spectrum system as compared to more conventional non-spread spectrum systems. Because of this complexity, spread spectrum systems typically use predefined chipping or hopping rates and thus cannot vary the spread spectrum encoding to adapt to varying requirements or changing conditions. Moreover, existing spread spectrum systems are typically designed to address one TRANSEC threat (e.g., jamming) and do not necessarily perform well when faced with a different type of TRANSEC threat (e.g., geolocation).